Well at points Biggles I have seen many times that Halo Reaches population was higher than Halo 4s population and one time I even remember Halo 3s being higher than Halo 4s so I would say they all have a decent population.
UNSC Spartan II let me try and make the answer easier to understand...
This answer is not part of the Halo lore at all it could be anything outside of the lore.
( Sorry if it is vague but it should narrow down your guesses by a lot )
My first post > the reason they change couluer is when the chief lands on requilm they attck him then when they are blue are amoutead defense sysmthem laster when there oronge they are under controll of the didact
Jl I was being sarcastic .....
Did any of you really think that kill zone fan boy was the riddle answer ?
Now for the clue ...
The answer to this riddle is not in Halo lore...